Hydrogen sulfide is a toxic, corrosive and malodorous compound. It may be found in a variety of liquid and gaseous media such as natural gas, petroleum, refinery gas streams, carbon dioxide, hydrogen, coal gas streams, gas streams from viscose rayon production, tars and asphalt, shale gas, coke oven gases, ammonia synthesis gas, rubber vulcanization streams, gases from sulfurization plants, turpentine production, pulp and paper mill effluent, sewage, brine drilling mud, land fills, phosphoric acid production gas streams, and other industrial gas streams and effluents. It is also found in the tail gases and liquids of some hydrogen sulfide scrubbing processes such as Claus plants and amine scrubbing units.
Hydrogen sulfide is an undesirable contaminant and its release into the environment is strictly controlled by the Environmental Protection Agency, the Department of Environmental Resources, as well as by other regulatory agencies throughout the world. Hydrogen sulfide not only has an offensive odor, but it has also been linked to the formation of acid rain.
Methods for removing hydrogen sulfide may be generally classified as regenerative and non-regenerative. Regenerative processes are generally more desirable because waste products are recycled. By regenerating sulfur scavenging compounds and thereby recycling the waste products, the cost, both economically and environmentally, of replenishing spent chemicals in the process and disposing of the waste products is lessened or eliminated. It is even more desirable to recover the sulfur scavenged during the hydrogen sulfide scavenging reaction in a useful form.
Various amines and alkanolamines, which may be regenerated, have been used to remove acids, such as hydrogen sulfide from gas streams. U.S. Pat. No. 2,776,870 discloses that aqueous amines and alkanolamines are useful for removing acids from a gaseous mixture. Hydrogen sulfide may be selectively removed from gas streams containing carbon dioxide by use of triethanolamine or methyldiethanolamine.
British Published Patent Specification No. 2103645 discloses that hydrogen sulfide and carbon dioxide may be removed from a gas mixture by contacting the mixture with a solvent comprising a tertiary amine and a physical absorbent. Suitable physical adsorbents include N-methylpyrrolidone and sulfolane.
U.S. Pat. No. 4,112,051 discloses a process for removing acidic gases from a gaseous mixture with an amine-solvent liquid absorbent comprising (1) an amine comprised of at least about 50 mole percent of a steric hindered amine; and (2) a solvent for the amine mixture which is also a physical absorbent for the acid gases. Suitable steric hindered amines include various piperidine compounds. Suitable solvents include sulfones and pyrrolidone and piperidone compounds, to name a few.
U.S. Pat. No. 4,978,512 discloses methods for reducing the levels of hydrogen sulfide and organic sulfides in a hydrocarbon stream by contacting the stream with a composition comprising the reaction products of a lower alkanolamine with a lower aldehyde. Suitable reaction products include mixtures of triazine and bisoxazolidine compounds.
U.S. Pat. No. 4,647,397 discloses a process and composition for removing hydrogen sulfide and similar sulfides from a gas stream. The gas stream is contacted with a substituted aromatic nitrile having an electron-attracting substitutent on the aromatic ring at least as strong as a halogen and an organic tertiary amine in an inert organic solvent, such as N-methyl-2-pyrrolidone. The spent contacting solution may be regenerated by heating the solution above the decomposition temperature of the reaction products to separate the sulfides from the liquid phase absorbent solution.
U.S. Pat. No. 4,775,519 discloses a continuous process for removing acid gas components from a gas stream by counter-currently contacting the stream with an aqueous solution of a mixture of N-methyldiethanolamine (MDEA) with imidazole or a methyl substituted imidazole. The gas is de-absorbed from the MDEA and the imidazole by reducing the pressure and causing the gas to flash.
U.S. Pat. No. 4,624,838 discloses a process for removing acid gases from a gaseous stream by contacting the stream with an aqueous scrubbing solution containing a hetero amine comprising either a five- or six-membered ring having a pKa no greater than about 8. Preferred hetero amines include imidazole and piperazine compounds.
U.S. Pat. No. 5,128,049 discloses a method for reducing the hydrogen sulfide content of hydrocarbon-containing fluids and aqueous solutions by injections of a dilute solution of a scavenging agent. Suitable scavenging agents include hexahydro-1,3,5-tris(2-hydroxyethyl)-s-triazine and various other compounds.
U.S. Pat. No. 5,347,003 describes a regenerative method where an N--C--N compound is regenerated from a product of a sulfur scavenging reaction, in which said N--C--N compound removes a sulfur atom from a sulfur compound to form the original N--C--N compound.
U.S. Pat. No. 3,622,273 discloses a regenerative method for the removal of hydrogen sulfide from a gaseous stream wherein the gaseous stream is contacted with a solution containing, by weight, from 0.005 to 20 percent of a ferric ion complex, from 25.0 to 99.945 percent of water and from 0.05 to 10.0 percent of a buffering agent selected from the group consisting of alkali metal carbonate.
There are numerous patents dealing with the removal of hydrogen sulfide from liquid or gas streams with various metal chelates through redox reactions with a higher oxidation state of the metal followed by oxidative regeneration through the use of air. As a sampling: U.S. Pat. No. 4,076,621 deals with iron chelates for the removal of hydrogen sulfide from water; U.S. Pat. No. 4,414,194 deals with iron chelates with alcohol as a crystal modifier; U.S. Pat. No. 4,436,714 deals with the use of metal chelates followed by electrolytic regeneration. All of the patents related to the use of metal chelates have in common the use of the metal ion to directly oxidize hydrogen sulfide to a higher oxidation state. A draw back of this technology is the long contact time required in order to achieve efficient removal of hydrogen sulfide from the gas streams and the cost of the reactants. Another drawback is the tendancy of the metal chelates to precipitate out of solution with pH changes or over time due to the relative instability of the chelated metal ions.
U.S. Pat. No. 4,455,287 describes a method of oxidizing hydrogen sulfide in gases to elemental sulfur by a continuous polyvalent metal catalytic liquid phase oxidation with catalysts such as iron chelate solutions in which the solution is stabilized by incorporating a general purpose biocide at a level below the kill concentration of the biocide. Typical biocide concentrations were in the range of low parts per million by weight of solution. This is different from the present invention in that the aminal compounds are not general purpose biocides, although at least one has been used as a fungicide. None of the biocides described are known to reduce the levels of hydrogen sulfide. Additionally, the levels of biocide used were far too low to achieve a measurable reduction of hydrogen sulfide, even if they were reactive.
There is a long-felt need in the art for improved, economic methods for regenerating sulfur scavenging compounds. Regenerating such compounds is not only environmentally desirable, but is also cost efficient and may reduce or eliminate the need for expensive processing equipment. In addition, since the scavenging compounds are regenerated, the need for purchasing replacement scavenging compound is greatly reduced.
An even greater benefit may be realized when the scavenged compounds are converted to a commercially useful form. The regeneration of sulfur compounds in such form provides a further financial incentive for the scavenging of such compounds and even further reduces the burden on waste disposal systems.